cgroup_calc_subtree_ss_mask() currently takes @cgrp and
@subtree_control.  @cgrp is used for two purposes - to decide whether
it's for default hierarchy and the mask of available subsystems.  The
former doesn't matter as the results are the same regardless.  The
latter can be specified directly through a subsystem mask.

This patch makes cgroup_calc_subtree_ss_mask() perform the same
calculations for both default and legacy hierarchies and take
@this_ss_mask for available subsystems.  @cgrp is no longer used and
dropped.  This is to allow using the function in contexts where
available controllers can't be decided from the cgroup.

v2: cgroup_refres_subtree_ss_mask() is removed by a previous patch.
    Updated accordingly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>